In some respects at least, how well a pilot performs in flight depends upon the ability to plan and coordinate the use of the power and flight controls for changing the forces of thrust, drag, lift, and weight. It is the balance between these forces which the pilot must always control. The better the understanding of the forces and means of controlling them, the greater will be the pilot's skill at doing so.

   First, we should define these forces in relation to straight and level, unaccelerated flight.

   Thrust is the forward force produced by the powerplant/propeller. It opposes or overcomes the force of drag. As a general rule it is said to act parallel to the longitudinal axis. However, this is not always the case as will be explained later.

   Drag is a rearward, retarding force, and is caused by disruption of airflow by the wing, fuselage, and other protruding objects. Drag opposes thrust, and acts rearward parallel to the relative wind.

   Weight is the combined load of the airplane itself, the crew, the fuel, and the cargo or baggage. Weight pulls the airplane downward because of the force of gravity. It opposes lift, and acts vertically downward through the airplane's center of gravity.

Lift opposes the downward force of weight, is produced by the dynamic effect of the air acting on the wing, and acts perpendicular to the flightpath through the wing's center of lift.
 

In steady flight the sum of these opposing forces is equal to zero. There can be no unbalanced forces in steady, straight flight (Newton's Third Law). This is true whether flying level or when climbing or descending. This is not the same thing as saying that the four forces are all equal. It simply means that the opposing forces are equal to, and thereby cancel the effects of, each other. Often the relationship between the four forces has been erroneously explained or illustrated in such a way that this point is obscured. Consider Figure 17-14, for example. In the upper illustration the force vectors of thrust, drag, lift, and weight appear to be equal in value. The usual explanation states (without stipulating that thrust and drag do not equal weight and lift) that thrust equals drag and lift equals weight as shown in the lower illustration. This basically true statement must be understood or it can be misleading. It should be understood that in straight, level, unaccelerated flight, it is true that the opposing lift/weight forces are equal but they are also greater than the opposing forces of thrust/drag that are equal only to each other; not to lift/weight. If we are to be quite correct about this matter, we must say that in steady flight

      a. The sum of all upward forces (not just lift) equals the sum of all downward forces (not just weight).
      b. The sum of all forward forces (not just thrust) equals the sum of all backward forces (not just drag).
 

This refinement of the old "thrust equals drag; lift equals weight" formula takes into account the fact that in climbs a portion of thrust, since it is directed upward, acts as if it were lift, and a portion of weight since it is directed backward acts as if it were drag. In glides, a portion of the weight vector is directed forward, and therefore acts as thrust. In other words, any time the flightpath of the airplane is not horizontal, lift, weight, thrust, and drag vectors must each be broken down into two components (Fig. 17-15). Discussions of the preceding concepts are frequently omitted in aeronautical texts/handbooks/manuals. The reason is not that they are of no consequence, but because by omitting such discussions, the main ideas with respect to the aerodynamic forces acting upon an airplane in flight can be presented in their most essential elements without being involved in the technicalities of the aerodynamicist. In point of fact, as long as we consider only level flight, and normal climbs and glides in a steady state, it is still true that wing lift is the really important upward force, and weight is the really important downward force.

   Frequently much of the difficulty encountered in explaining the forces that act upon an airplane is largely a matter of language and its meaning. For example, pilots have long believed that an airplane climbs because of excess lift. This is not true if one is thinking in terms of wing lift alone. It is true, however, if by lift we mean the sum total of all "upward forces." but when we begin to talk about the "lift of thrust" or the "thrust of weight," the definitions we have previously established for these forces are no longer valid and we have simply further complicated matters. It is this impreciseness in language that affords the excuse to engage in arguments, largely academic, over refinements to basic principles.

   Though we have already defined the forces acting on an airplane, we should discuss them in more detail to establish how the pilot uses them to produce controlled flight.